In the conventional manner for manufacturing sulfuric acid, sulphur is oxidized in the presence of heat and air and/or supplemental oxygen to form SO.sub.2 which is then passed through a converter containing vanadium pentoxide catalyst to convert the same to SO.sub.3. The gaseous SO.sub.3 is then contacted with concentrated sulfuric acid to form additional sulfuric acid. The gaseous effluent from this operation contains SO.sub.2, SO.sub.3 and a mist commonly called sulfuric acid mist. This sulfuric mist is formed when SO.sub.3 is contacted with moist air and forms small particles of sulfuric acid.
In order to remove these components from the effluent gaseous stream, the gases are contacted with a series of scrubbers and/or absorbers that essentially remove all the SO.sub.2, SO.sub.3 and sulfuric acid mist. Since these systems are not 100% efficient, from about one to twenty milligrams of sulfuric acid mist per cubic foot of effluent gas has heretofore been ejected into the stack and into the atmosphere. When this occurs, a rolling plume of the gaseous mist emits from the stack and is visible as a smoky cloud.
With the advent of more stringent government regulations for the removal of undesirable emissions and in an effort to improve community relations, it has become necessary to remove essentially all of the acid mist from being emitted. For this purpose a mist eliminator system has been devised. This system essentially consists of a large tank structure with a partition between the inlet and outlet wherein mist eliminator units are installed. The most commonly used mist eliminator consists of a vertical packed fiber bed retained between two screens. The elements may be a fiber bed packed between two concentric cylindrical screens or between two parallel flat screens. Chemically resistant fiberglass, synthetic fibers and other special type fibers are used for packing material, depending on the environment. Structural parts are made of any wrought weldable metal or glass reinforced resins. The exhaust gases containing the mist particles pass in a horizontal direction through the fiber bed. Clean gases emerge from the bed and rise upwardly to the exit from the system. The liquid particles are collected on the fibers in the bed and coalesce into liquid films which are moved horizontally through the fiber bed by the drag of the gases and then downward by gravity. The collected liquid drains off the downstream face of the fiber bed and out through the drain legs. If the elements are installed in a separate tank or vessel, the drain legs are immersed below a liquid level in the bottom of the tank to maintain a liquid seal. If the mist eliminator elements are suspended in the top of an absorbing tower, the drain legs are fitted with individual seal cups to maintain a liquid seal. One such mist eliminator system is sold under the trade name Brink.RTM. mist eliminators by Monsanto Enviro-Chem Systems, Inc., of St. Louis, Missouri. For a more detailed explanation of the function of this type of mist eliminator, reference is made to a technical bulletin identified as IGI LRA 0472/10M/B08. While the design and construction of this type of mist eliminator forms no part of this invention, the subject matter of this technical bulletin is incorporated herein by reference for completeness of the description of the subject matter of this invention.
Under normal sevice conditions, the above described mist eliminators remove 99.5% or more of all sulfuric acid mist from the gaseous stream prior to emission from the exhaust stacks. However, when an upset conditon upstream of the eliminator unit exists and a large volume of SO.sub.3 gas is injected into mist eliminator unit, it has been found in practice that these mist eliminators will not function to remove the SO.sub.3 gas. The upset condition can occur when the plant is in a start-up condition or more commonly when an absorber or a scrubber upsteam in the mist eliminator unit malfunctions due to mechanical difficulties. Under these circumstances, large volumes of SO.sub.3 gas is injected into the mist eliminators. When these large volumes of SO.sub.3 gas are injected into the mist eliminator unit, they quickly saturate the units with the SO.sub.3 gas which forms oleum and a breakthrough occurs and substantial amounts, by volume, of the SO.sub.3 gas is then emitted through the stacks into the atmosphere. Thus a large rolling plume of smoke appears in the atmosphere which is clearly visible and undesirable.
In order to rectify this situation, a plant shut-down is required to not only repair the malfunction but also to remove all the SO.sub.3 gas from the mist eliminator units. This latter operation requires that the mist eliminator unit within the tank be flooded with sulfuric acid. This amounts to filling the tank with sulfuric acid to absorb the SO.sub.3 gas from the eliminator units and then draining the same. This operation requires from six to twenty-four hours to carry out.